Omegatron with orbit increment detection



Nov. 1, 1960 1.. R. MONARRY EI'AL 2, ,7

OMEGATRON WITH ORBIT INCREMENT DETECTION Filed May '7, 1957 2Sheets-Sheet 1 ORBITS OF NON-RESONT IONS I I i l l A AM'INVENTOR*LEONROBERTMc/VARRV JOH N PETER HOBSO/V AGENT Nov. 1, 1960 -1 R. McNARRYEI'AL OMEGATRON WITH ORBIT INCREMENT DETECTION r wmmm RESONANT IONORBITS WW/U)! I AC FIELD NON RESONANT ION ORBITS IN VENTOR t 0/v ROBERTM: NARRY 4 kc/s JOHN PETER HOBSO/V SI NG LE COLLECTOR DUAL ELECTRODEOMEGATRON j!- 0 8 KC 7 mOPuwjOu O. .rZwEEDU (4) 25,4 kc/S (1.) 25,4 kc/sAGENT United States Patent C OMEGATRON WITH ORBIT INCREMENT DETECTIONLeon Robert McNarry, Cumberland, Ontario, Canada,

and John P. Hobson, Cardinal Heights, Ontario, Canada, assignors toNational Research Council, Ottawa, Ontario, Canada, a body corporate ofCanada Filed May 7, 1957, Ser. No. 657,616

4 Claims. (Cl. 250-419) This invention is in improvements in apparatusfor analytically separating ions on the basis of differences in theirmass-to-charge ratio, and is particularly concerned to provide novelelectrode dispositions in omegatron apparatus to improve resolution overa wider range of specific mass numbers of substances than has heretoforebeen possible.

Mass spectrometry is generally concerned with spatial separation ofions, the technique being based primarly on the inherent differences inmotional behaviours of heterogeneous ions of a sample of a substance tobe analyzed, when subjected to the influences of a magnetic or electricfield or both. It has been found that particularly effective spatialseparation as a function of the specific masses of ions is possible whenthe ions move initially from rest along a line source across a magneticfield while being subjected to a high frequency alternating electricalfield in which the electric vector is normal to the magnetic vector. Insuch combined field the ions move in spiral paths about axes of gyrationparalleling the magnetic vector, the gyratory movements being uniquelycharacteristic of each combination of field intensities, electric fieldfrequency, and the mass-to-charge ratio of the ion. When a certainrelationship of magnetic and electric intensities exists in the combinedfield and the alternating electric field frequency is non-varying, ionsof a predetermined specific mass will be sped along a spiral path ofsteadily increasing radius. a manner closely approximating thereto arereferred to as resonant ions. All other ions of different specific masswill travel in curved paths whose radius at first increases at anon-uniform rate to a maximum orbit diameter, thereafter decreasing at anon-uniform rate to return to the axis of origin. Such other ions arereferred to as non-resonant ions. Accordingly, the mass spectrometer ofthe latter type can make a spatial separation by constraining all ionswhich are non-resonant under the effects of the applied fields tomovements within a certain radius of the origin, while resonant ions arecollected by a collector electrode placed at a larger radius from theaxis.

The line source or origin of ions may be realized by propelling aslender ionizing beam of high speed electrons along the magnetic vectordirection, or by injecting the sample as a slender ionized line beamalong such direction, or by passing a slender rod-like beam of ionizingradiation along such axis through the chamber in which the rarefiedsample is released. The axis of origin is ideally made to have as smalla cross-section as possible in order to minimize deviations fromtheoretically perfect operation in which all ions start from rest alonga straight line source.

The omegatron apparatus described may be adjusted in turn to energizeions of each mass number to resonance, by varying the frequency of thealternating electrical field while maintaining the magnetic field andthe RMS voltage of the electric field constant. As ions of each massnumber in turn are carried beyond their nonresonant orbit path diameterthey are collected and de- Ions behaving in this manner or in' 2,958,774Patented Nov. 1, 1960 2 tected by measuring the current delivered to thecollector electrode.

The motional behaviour of non-resonant ions whose specific masses nearlycorrespond with the mass of a resonant ion for given steady conditionswithin the analyzer region is very closely similar to that of theresonant ions until a number of revolutions have been made by each aboutthe axis of origin. It is only after the radius of the spiral paths hasexceeded the value at which nonresonant ion orbits collapse that theseparation is effected, and for large mass number ions the resolution inconventional omegatron mass spectrometers is poor while the requiredcross-section of an analyzer space is large.

In practical form, an analyzer apparatus employing a steady magneticfield of high intensity and large crosssectional area is very costly andpresents difficulties in achieving uniformity of field and steady fiuxdensity. A further disadvantage of conventional omegatron massspectrometers as devices discriminating between ions of large massnumber arises from the lengthening of the path with increase in thenumber of revolutions required to be made by the mingled resonant andnon-resonant ions before separation is effected, during which spacecharge effects disturb the paths of resonant ions and impair resolution.

Heretofore a reduction in the area of the required magnetic field hasbeen achieved either by intensifying the field, or by employing anorbit-distorting steady electrical field transversely of the line axis,as set out in the specification of United States Patent 2,718,595 to C.F. Robinson. A spectrometer as described in the aforesaid publicationincorporates a scavenging electrode spaced diametrally opposite to acollector electrode to remove ions and diminish space charge, whileresonant ions increase their orbit radius against the steady D.C. fieldto arrive at the collector electrode.

None of the prior art devices have good resolving power fordiscrimination between ions of adjacent large mass numbers, particularlyfor mass numbers of the order of and higher. Such prior art devices arerelatively ineffectual in discriminating between ions of adjacent largemass numbers, such as heavier elements and molecular fractions of ioniccharacter.

Applicants have realized a novel and greatly improved mass spectrometerdevice employing the combined magnetic and varying electric fields asmentioned above, which differentiates between ions according to a newprinciple, enabling resolution of a high order to be achieved indetecting and identifying ions over a very broad range of mass numbers,without any increase in the size of analyzer chamber or magnetic fieldintensity.

As compared with prior art devices, separation of ions of adjacent massnumbers which may be of the order of several hundred, is effected with arelatively reduced area and intensity of magnetic field and in fewercycles of orbital travel around the line axis, by virtue of a novelcollector system employing two electrodes.

Applicants have established a novel theory of spatial discrimination andhave developed improved mass spectrometer apparatus of the omegatrontype embodying the invention, employing an arrangement of dualelectrodes between which the ions normally caught by a single collectorare divided. The novel omegatron utilizes the property of the ion orbitsthat the resonating ions have a greater increment in radius in a singlegyration than any non-resonant ions. Non-resonant ions and a certainproportion of resonant ions are caught upon an interceptor electrode ata lesser radial displacement from a source region than a collectorelectrode upon which only resonant ions are caught. In a massspectrometer organization as described which is operated atpredetermined magnetic and electric field intensities, theresolutionisoptimum when the increment of radius between the inner edge of theinterceptor electrode and the corresponding edge of the collectorelectrode is made not greater than or fractionally smaller than theincrement of radius per revolution of those ions resonating at theapplied field frequency 'The invention permits the use of much broadercrosssections of ionized beam sources than have hitherto been operable,since the restriction to a fine line source imposed on conventionalomegatrons is removed in the double collector omegatron.

According to the invention a mass spectrometer is real- 'ized as anevacuated envelope within which a specimen of substance to be analyzedis released in gaseous form, enclosing an electrode structure andsupports for setting up a transverse electric field crossing a magneticfield applied externally, and a means to introduce an ionizing beam ofelectrons along an axis aligned with the magnetic field through theanalyzer chamber, there being provided an interceptor electrode spacedfrom the beam axis and an adjacent collector electrode at a largerradius and disposed within the region pervaded by the crossed magneticand alternating electric fields on that side of the interceptor impingedby accelerated ions during operation.

In carrying the invention into effect an analyzer chamber is disposedwithin a non-magnetic and gas impermeable vessel which may have separatespatial connections with an evacuating system and with a source ofsubstance to be analyzed. Alternatively no special tube for introducinga sample need be provided where analysis of residual gases is to bemade. The chamber is axially short and a steady magnetic field ofuniform intensity is applied along the axis. A beam of ionizingelectrons is produced by accelerating them along the chamber axis from acathode source by an electron gun to pass through the chamber to anelectrode spaced across the chamber and generally disposed centrally ofthe magnetic field. A system of electrodes is arranged to provide, whenexcited by applying alternating voltage from a source of adjustableintensity and frequency, an electric field transversely of the magneticfield and of substantially uniform strength throughout at any instant.

The invention may be the better understood by study of the descriptionwhich follows, together with the accompanying drawing, wherein:

Figure 1 is a diagram representing half cycles of the theoreticalmovements of a resonant ion in a conventional omegatron type of massspectrometer as projected .on a plane normal to the axes of gyration,and similarly the movements of a non-resonant ion of adjacent massnumber.

Figure 2 is a diagram of a portion of the ion trajectories of Figure lin enlarged scale, showing the relative positions of an interceptorelectrode and a collector electrode according to the invention, and theradial increments of ion orbits;

Figure 3 is a three dimensional sketch showing the arrangements of acomplete omegatron including the electrode structure of the invention;

Figure 4 is a diagram describing the motions of resonant andnon-resonant ions of adjacent mass numbers in a mass spectrometer havinga steady field applied along the Z axis in addition to the combinedmagnetic and alternating electric fields, and the location of a pair ofelectrodes according to the invention.

Figure 5 is a diagram graphically comparing the resolving powers of anomegatron employing an interceptor electrode adjacent to and spacedradially inwardly from a collector electrode, for ions of mass number200 in one case as a two-electrode system, and in the other as aconventional single electrode omegatron; and,

Figure 6 is another graph diagram to illustrate the dependence ofresolving power on increment of radial distances between the inneredges. Qfi h interceptql and the collector electrodes.

with return of the ion to its source. be collected on an electrode 10placed at'a radial distance Referring to Figure 1, the paths of ions inthe X2 plane are shown as originating at an axis of origin Y of a systemof three mutually orthogonal axes X, Y, Z, the Y axis beingperpendicular to the plane of the drawing and drawn parallel with thedirection of the magnetic vector. An alternating electrical field hasits electric vector parallel to the Z axis throughout the orbital regiondesignated in which the magnetic field is of uniform intensity.Half-cycles of the orbits of an ion of a given mass number resonant atthe applied frequency w are traced above the 'X axis, these exhibiting auniform and constant increase inorbit radius for each crossing of the Xaxis to the left of the origin. For purposes of comparison the path ofan ion of adjacent mass number not resonating at the frequency of theapplied alternating field is traced below the X axis; consecutivegyrations of these ions are in paths increasingly distant radially fromthe origin, the increase being at a non-uniform and diminishing rate,gradually becoming zero at an outer radial limit, then becoming negativeas the orbit collapses Resonant ions may beyond the non-resonant ionmaximum orbit. The dia gram is general and applies to ions of any massnumber, with appropriately related dimensions and field parameters. Itwill be apparent that for the spatial separation of ions of mass numbersof the order of 200 and higher, very large magnets are required tosustain a field through a correspondingly large diameter analyzerregion.

At Figure 2 the arrangement of a pair of electrodes 11 and 12 withrespect to the arbitrary paths of a resonant ion and of a non-resonantion may be examined. This figure shows in an enlarged scale a portion ofconsecutive half cycles tracks 13, 13 and 13 of a resonant ion drawnabove the X axis, and similar portions of half cycles 14, 14 and 14" ofnon-resonant ions of adjacent mass number, below the X axis, the originY lying to the right of the figure. The omegatron combined fields areapplied as has been described for the Figure I diagram.

Let it be assumed that a resonant ion expands its orbit from track 13 atradius r to trace the paths 13, 13 in its consecutive similar halfcycles, with uniform increments Ar, and that path 13 clears the edge ofinterceptor electrode 11 by a distance 8. The radius distances from theaxis of origin Y measured to the near edges of electrodes 11 and 12 arerespectively R and R units, where In order for collision of an ionmoving in path 13" to occur with electrode 12 there must be therelationship:

The quantity 6 is assumed completely random for any ion, depending inpart on its point of origin and number of gyrations executed since itstarted. This stems from the fact that the source of ions never conformsto an ideally thin line axis of origin. When an intense, broadenedsource of ions is employed, as may be required for the quantitativeseparation of element isotopes, ions may just clear electrode 11 afterhaving made widely different total numbers of gyrations around the axis.

The quantity Ar is controllable and for resonant ions it is preferablyso adjusted that Ar-AR 2,0

It will be apparent that if ArAR 0 no ions would ever reach collector 12and the device would function as a single electrode or conventionalomegatron.

In the omegatron device employing the dual electrodes 11 and 12, allions whether resonant or non-resonant whose orbits intersect electrode11 are collected, thereby removing a population whose space chargeeffects would otherwise tend to disturb resonant ion orbits prior tocollection.

A non-resonant ion gyrating at radius r and just 'clearing the near edgeof electrode 11 in its track 14 so that will have a Ar which is alwaysless than Ar for resonant 'ions. Consequently, for ion orbits at least afew cycles removed from their point of origin,

so that the ion collides with electrode 11 in its next consecutive orbit14" and fails to reach collector electrode 12. It will be apparent thatthe swarms of ions streaming in the vicinity of electrode 11, withapproach orbits of random 6 and distances will divide themselves betweenthe near electrode 11 and the further collector electrode 12. A certainproportion of resonant ions will have 6 values such that in their nextgyration they impinge on interceptor electrode 11 and fail to contributeto the measured ion current. However as will be directly apparent, onlya resonant ion is capable of gyratory motion such that if it just missesbeing intercepted by electrode 11, in its next cycle it will impinge onelectrode 12.

The starting point of ions need not be restricted to a line source sinceregardless of initial radius distances r or r, both resonant andnon-resonant ions will approach the electrodes in their next-to-lastorbits with random 6 andn 6' clearances. Therefore a greatly intensified0r broadened source such as a tubular or columnar ionized zone may beemployed with a correspondingly large increase in ion population. Theelectrode disposition moreover is not restricted within the analyzerregion as to minimum radius from the source, since only a very fewcycles of gyration need be executed by ions before they are sufficientlyremoved from the source to manifest different orbit increase distances.

A great reduction in magnet cross-section area is gained by thisfeature. Moreover the device operates to continuously intercept ions andremove them from circulation after a minimum history of gyration. Aswill be shown hereinafter, the required diameter may be only a fewpercent of that for a conventional omegatron of the same resolvingpower.

The breadth of the source may in some instances advantageously be made aconsiderable fraction of the analyzer space diameter, the electrodespacing AR being adjusted along a radius drawn through the center of thesource zone. For resonant ions whose orbits are not substantially normalto this radius the effective AR is less than its measured value; as alimitation to the breadth of the source zone and the radial spacing ofthe electrodes therefrom, the effective AR must always be larger thanthe Ar of non-resonant ion orbits.

The motion of an ion in the crossed fields of an omegatron analyzerregion is mathematically described in the analysis which follows, basedon the diagrams of Figures 1 and 2. An electrical field E is assumed tobe directed along the Z axis of Figures 1 and 2, produced as by applyingan alternating voltage between parallel plates (not shown), so that atany point therebetween:

E=E0 Sin wt A magnetic field of intensity B is assumed to be appliedalong the Y axis of the diagrams, the vector direction being in suchsense that ions move clockwise in these diagrams. Ions are formed alongthe Y axis, moving outwardly in the XZ plane from rest at time t=0. Theinitial conditions are: 7

Since there are no forces on the ion in the Y direction the motions inthe XZ plane only need to be con- Sidered. 1 i

6 The equations of ion motion in the crossed electric and magnetic fieldis given by the expression:

=w ditldt+w Eo sin cot/B where (e/m) is the ratio of ion charge to itsmass,

d x/dt is'the acceleration of the ion in the X direction,

d z/ d! is the acceleration of the ion in the Z direction,

dx/dt is the velocity of the ion in the X direction,

dZ/dt is the velocity of the ion in the Z direction, and w is theangular frequency of the applied voltage having a peak voltage E Theintegration of Equations 2 and 3 and the application of Condition 1'yield the solutions:

Z =F (w sin wtw sin w t) (5) as may be verified by substitution.

From a considerationof the displacement Functions 4 and 5 as expressedin the Cartesian co-ordinate system, it may be shown that the radialposition of any ion at any time t is derived from the solution ofcomponents of a right-angle triangle "(0 t t is where r is the radiusmeasured from the Y axis of origin.

Substituting the Solutions 4 and 5 into this expression and using theapproximation that near ion resonance Where Am is the difi'erencebetween the resonant frequency of an ion and the alternating frequencyof the applied electric field. The expression also may be written:

The rate of orbit increase is found by differentiation of Equation 6with respect to t:

Applying Equation 7,

The time for one complete gyration of an ion about the Y axis is:

1 21r 21r m 7 7 7(?) At orbit radius R, the general expression for ionorbit increase may be defined by allowing Letting Aw 0, i.e.establishing resonance a resonant ion.

increases its orbit radius per cycle by the amount Ar 'H'Eg m From anexamination of the form of the Equations 8- and 9, it will appear thatthe increment of orbit radius for a resonant ion is maximum andconstant, increasing with the RMS field magnitude and increasing withthe mass-to-charge ratio of the ion, while the increment diminishes asthe square of the magnetic field intensity. Hence for resonant ions ofany mass number to undergo uniform orbit expansions per cycle in theanalyzer system, the applied voltage must be so related that where K'isa constant. Ions detuned from the frequency of the applied voltage haveion orbit increases which equal the resonant ion Ar only at thecommencement of their gyrations very close to the Y axis where R issmall; however with the expansion of orbits to increased R values alimiting radius is reached where i i The total spread or range ofapplied field frequencies for a single collector omegatron may thus bedemonstrated to be ZAw which may be written 8 The resolving power of theapparatus may be defined as the ratio which is expressible in thestandard form In Figure 6 a horizontal line is drawn through'the diagramat an arbitrary ordinate distance AR, correspond ing to a radialspacingof two electrodes within the analyzer region. In accordance withthe discussion for Figure 2, no ions can reach collector 12 unless theirorbit increase distances exceed AR. Equation 8 may be rewritten into theform expressing a circle having its center. at 0, and ordinate andabscissa values proportional to Ar and Aw respectively:

For each arbitrary AR and a corresponding maximum 5 for resonant ionorbits, it will directly appear that ions resonating at frequencies inthe range Aw to +Aw will be collected on collector 12. The frequencylimit A012 may be calculated by substitution into Equation 8:

AR 12 5mm ree 0 from which An expression may now be derived for theresolving power R of a two-electrode omegatron:

2E0 AR 1 res Substituting from Equation 10,

AR 2 l -(mt) e R B AT m2 DI: 1'85) 2 A 1 and substituting from Equation9,

i Rx: res) 2 P Equation 12 defines the resolution capability of atwocollector omegatron in terms of location at radius R, magnitude ofresonant ion orbit increment, and radial separation of electrodes. Itshows that the resolving power increases as the square root of thedifierence between the squares of Ar and AR, being controllable byadjustment of field intensities and by geometry of design. The ultimatediscriminatory power of such omegatron device is indicated when Ar AR AtAR=0, i.e. for the condition that a single collector confronts themoving ions,

The mass analyzing instrument shown in Figure 3 of.the. drawing.includes an analyzer chamber 15 having a housing 16 preferably of glass,enclosing an electrode structure and having plane parallel opposed wallsdisposed between poles 17 of a magnet. Suitable means are provided toadmit a gaseous sample of substance to be analyzed, as by port 18, and atubulation 19 communicating also with the chamber is adapted to connectthe system to a device capable of evacuating the chamber to a high orderof vacuum.

, An electron beam generating source 20 is contained in an electron gun,wherein an accelerating electrode 21 is at ground potential and theheated filament cathode 22 is at a high negative potential. A targetelectrode 23 spaced across the chamber from the cathode receives thebeam and may be at a slight positive potential, suitable electricalcurrent sources such as batteries 24, 25 being provided.

A pair of high-frequency electric field-forming plates 26, 27 aredisposed across the chamber and are parallel with each other, beingconnected with the output terminals of a variable frequency oscillator28 whose output voltage may be controlled and whose frequency isprecisely determinable. An electrode structure comprising an interceptorelectrode 11 and a collector electrode 12 are spaced a small distanceapart and are generally aligned parallel and along a radial line drawnthrough the locus of the ionizing electron stream between the gun 20 andthe target 23. Means are provided to bias the interceptor electrodesuitably, and to measure the ion current delivered to the collectorelectrode 12 as by a galvanometer 29 capable of reading very lowcurrents.

The operation of the mass analyzer may be described as follows: gaseoussamples of substances to be analyzed and identified as to constituentsare admitted by way of port 18 and allowed to diffuse into the chamber15, to a predetermined low gas pressure. The application of theoperating potentials to the gun 20 and the target 23 causes a beam ofelectrons to be formed as they traverse the chamber, colliding withgaseous molecules and ionizing them along a linearly extended region ofsmall cross section coaxial with the beam. The electron motion is alongthe magnetic intensity vector of the field pervading the region betweenpoles 17. Free ions formed by impacts are affected by the alternatingelectric field between the field-forming plates, being urged into spiralorbits around the beam, as described hereinbefore. Ions of mass numberand charge corresponding to the resonance relationship as set out inrelation are spiralled away from their point of origin, a certainproportion of these arriving upon collector electrode 12, while theremainder collide with and are removed by interceptor electrode 11. Nearresonant and certain of the non-resonant ions also are removed by theinterceptor electrode. A large number of non-resonant ions never reacheither collector and fall back into the source. A current indicated bydevice 29 represents the effect of ions of a selected mass numberdischarging to the electrode 12.

For low mass numbers, the device may be operated as a conventionalomegatron by connecting the electrodes 11 and 12 together as input tocurrent meter 29. Above mass number 20 it will generally be foundadvantageous to resort to the dual electrode connection with outerelectrode 12 serving as the collector. The operation of the device inanalyzing mass numbers above 100 is substantially possible only with theelectrode arrangement of the invention, and indeed the conventionalomegatron would fail as a discriminator for higher mass numbers, as maybe understood by comparing the current-versusfrequency measurementsindicated by Figure for mass number 200. In this figure, the Aw valuesare measured on each graph between the flanks of the current spikes atthe points where these rise above noise level, an improvement inresolution of five tim'es' being realized. The operating conditions inthis system were as follows:

The applied frequency, 25.4 kilocycles was applied to the system platesin each test. The spread Aw in the caseof the single collector omegatronis 4 kc./sec., which is of the order of 16% of the resonance frequency,indicating that detection of ions having mass numbers in the: range fromabout to 220 are masked by the observed. current. In terms of theresolving powers of the instru-- ment, the measured resolution of thetwo-electrode omegatron is 63, whereas that of the single collectordevice is only 13.

The resolution of a dual electrode omegatron according to the inventionmay be increased by varying certain parameters, as may be appreciated byinspection of Equation 12, discussed above. By increasing the radialdistance R of collector 12 from the origin, an improvement in resolutionmay be realized in direct proportion to increase in magnet diameter. Amore efficient approach to the problem of improving resolution is toreduce the magnitude of the denominator of-the expression to as small avalue as practicable. For a given AR, the resonant ion orbit increase Armay be reduced and the denominator thereby made to approach a very smallnumber, by the epedien-t of decreasing the magnitude of the appliedalternating electric field. As a practical limit to the improvementpossible by the latter means, the intensity of the ion current tocollector 12 eventually falls to a value at which it is comparable tothe level of circuit noise in the measuring system as a whole. It willbe clearly evident to those skilled in the art that any improvement innumber of ions per second liberated by the source will be reflected byan increase in signal current collected, permitting further adjustmentof system parameters to gain an increase in resolution.

An increase in measured current, with corresponding improvement in therelative sharpness of a current spike between flanks at average noiselevel value, is practicable as set forth, by enlarging thecross-sectional area of the ionizing beam. Ideally, a ribbon beam havingits Width aligned in the direction of the electrodes is preferable asminimizing the deviation of ion trajectories from normal incidence uponthe electrode system.

It is possible to incorporate a number of the prior art teachings forthe purpose of distorting the orbits for improved selection in certainzones of the analyzer, without diminishing the efiiciency of the dualelectrode omegatron as described. A unidirectional electric field may beapplied in addition to the alternating field, preferably the Z axis, toeffect a drift of ions along the X axis as shown in Figure 4. Aninterceptor-collector electrode pair 11,, 12 is placed in the chamber asfor Figure 3 embodiment. An increased AR radial separation is madepossible due to the increased Ar and Ar increments of ion orbitsresulting therefrom.

It has been shown earlier that for uniform Ar tobe achieved for ions ofany mass-to-charge ratio whose resolution is within the capabilities ofthe apparatus, the applied alternating electric field strength B shouldbe inversely proportional thereto. Accordingly, in order tomaintain aconstant resolving power for the system over at least a practicablerange of mass numbers the RMS value of the applied A.C. may be keptconstant and the strength of a DC. field reduced in accordance with thereduction of A.C. frequency. By this procedure the Ar, values ofresonant ions are kept substantially constant despite change in appliedA.C. field, the adjustment of drift being; relatively simply effected bycontrolling a DC. voltage 11 component applied to the field-producingAECL. electrode system.

The electrode pair 11, 12 need not necessarily be located in the regionof increased Ar orbit increments, and may be placed to the right of thesource in Figure 4, with a suitably decreased AR radial separation.

From the foregoing-itcan'be understood that a new and improved mass'analyzinginstrument is provided by the practice of the invention, havingvery considerably improved resolving-power, particularly for high massnumber ions. The device moreover provides for. reduced space chargeeffects despite operation withgreatly intensified ion generation means,with noreduction in the dis, criminatory capabilities of the instrument.Modification of the device to operate as a standard 'ornegratron at'lowmass numbers is inherently simple and direct.

It can be appreciated that many variations and forms of'the inventionmay be realized in the practice of the teachings herein set out. It isto be understood therefore that modifications and changes may be madefrom What has been described within the broadest scope of the inventionas defined by the appended claims.

We claim:

1. In a mass spectrometer of the ion-resonance type utilizing thecombined action of cross magnetic and radio frequency electric fieldsfor acceleration of ions into spiral orbits originating along an axialzone source of ions paralleling the magnetic field, the improvementcomprising a discriminatory collector electrode structure disposedwithin said fields and spaced laterally from said zone, including aplanar sensing electrode for collecting resonant ions and an adjacentplanar interceptor electrode coextensive with said sensing electrode,said sensing electrode being radially spaced at a greater distance fromsaid zone than said interceptor electrode. and lying in advance of saidinterceptor electrode with respect to moving ions, whereby saidinterceptor electrode. exposes a surface portion to be impinged both bynon-resonant and resonant ions, said portion having a radial spanmeasured in the range from a fraction of an orbit radius increment pergyration of a resonant ion to a distance substantially equal to but notexceeding one whole orbit radius increment.

2. In a mass spectrometer having an analyzer chamber, means foradmitting a sample to be analyzed into the chamber, means for developingions of the sample about the axis of a cylindric zone extending throughsaidchamber, means for developing a magnetic field across. the chamberparalleling the said axis, and means for establishing a high frequencyalternating electricfield across the chamber transversely of'themagnetic field whereby to urge ions into gyratory movement, theimprovement comprising a scavenging electrode and a collector electrode;said electrodes being axially coextensive and disposed within saidfields and spaced laterally from said zone axis, said collectorelectrode lying adjacently ahead of said scavengingelectrode withrespect to the direction of ion motion'and being laterally spaced agreater distance from saidvaxis than said scavenging electrode by adifference substantially equal to. but not exceeding the radius.increment. of theorbital pathof a resonant ionduring onecomplete'gyration about said axis, whereby said scavenging electroderemoves non-resonant ions and a fraction of saidiresonant ions, andsaidcollector electrode collects substantiallyzonly resonant ions.

3. In. a mass spectrometer, the combination comprising an analyzerchamber, means for developing ions of a sarnpleto be analyzed along anaxial zone extending through said chamber, means for developing amagnetic field across the chamber paralleling said zone, means forestablishing a high frequency alternating electric field transversely ofthe magnetic field for urging ions along spiralorbital paths about saidzone, a first planar electrode spaced laterally from said zoneand asecond planar electrode spaced radially outwardly from and disposedahead ofsaid first electrode in the paths of gyrating ions, theinnermarginal edges of said electrodes being parallel With said zone axis andthe second electrode being spaced further from said zone by a distancenot exceeding the radial increment of the orbit of a resonant ion forone complete gyration'about said zone, whereby said first electrodeintercepts and removes both non-resonant and resonant ions, and saidsecond electrode collects substantially only resonant ionswhose orbitsdo not interseotthe first electrode.

4. A high resolution mass spectrometer comprising an analyzer chamber,means for developing ions of a sample to be analyzed along an axial zoneextending through said chamber, means for developing a magnetic fieldacross the chamberparallel with the zone axis, means for establishing ahigh frequency alternating electric field transverselyof' the magneticfield for urging ions into gyratory motion, first and second electrodesspaced laterally outside saidzone within said fields and disposedadjacently of each other and having their inner marginal edges parallel.with said zone axis, said first electrode shielding .a portion of saidsecond electrode from impingement bymoving ions, and being spaced agreater'distance from said zone axis to provide an unshielded ionscavenge ing area adjacent the inner margin of said second elec: trode,said area having a radial extent not exceedingthe radius increment ofthe orbit of a resonating ion, whereby said second electrode interceptsboth non-resonant ions and a fraction of said resonant ions and saidfirst electrode collects substantially only resonant ions.

References Cited in the file of this patent UNITED STATES PATENTS2,627,034 Washburn Jan. 27, 1953 2,632,113 Berry Mar. 17, 1953 2,718,595Robinson Sept. 20,1955

